This invention relates to multi-hull seagoing vessels and in particular relates to high speed craft with three hulls that can be used to transport passengers and cargo in comfort whilst satisfying maritime stability standards.
International maritime regulations dictate the required stability of seagoing passenger and cargo carrying vessels. With multi-hulled vessels it is often the case that the compliance with the stability standards does not enhance the passenger comfort of the vessels.
It is this conflict between vessel stability and passenger comfort in multi-hull vessels that has brought about the present invention.
When floating at rest in still water, a vessel must obey the following natural conditions:
(i) the force of buoyancy, assumed to act vertically upwards, must equal the total mass of the vessel.
(ii) the point of application of the force of buoyancy, known as the centre of buoyancy, and the centre of gravity of the vessel must be in the same vertical line.
If a vessel is inclined to some small angle from a position of rest and when released it tends to return to the upright position it is said to be stable.
FIG. 1 shows a representative section through a ship inclined at some angle xcex8 to the vertical. The centre of buoyancy B in the upright position has moved to a new position B1. The vessel weight W acts downwards through the centre of gravity G, and the buoyant forces act upwards through B1. Consequently there is a couple tending to return the vessel to the upright position, where this righting couple is given by W.GZ, where the distance GZ is the righting lever. The righting couple can also be written as W.GM Sin xcex8 where M, called the metacentre, is the position of the intersection of the line of action of the buoyancy force acting vertically upwards, and the centre line of the vessel.
It is clear from FIG. 1 that the couple acts to restore the vessel to an upright position only when M is above G, and in this case the vessel is stable. If M is below G, then the couple will act to overturn the vessel, and it is unstable.
If M is above G, then the distance GM has a positive value, and it can be said that a vessel with a positive GM will be stable.
The righting lever GZ can be calculated from the geometry of the vessel together with the vertical height of the centre of gravity G. This can be done at various angles of heel of the craft to produce what is known as a GZ curve, illustrated in FIG. 2. It can be shown that a line drawn at a tangent to the GZ curve at zero angle of heel is equal to the value of GM at the position where the line intersects with an angle of heel of one radian (57.3xc2x0).
All vessels are required to meet a particular standard of stability. In many cases, and particularly for those vessels carrying passengers, the requirements are laid down by law. For vessels operating on a voyage between two countries, known as an international voyage, the regulations are formulated by the International Maritime Organization (IMO) by Resolution A.749(18), and published by the IMO in a booklet called xe2x80x9cCode on Intact Stability for All Types of Ships Covered by IMO Instrumentsxe2x80x9d, dated 1995.
These criteria include a requirement to meet a particular condition where the vessel is operating in severe weather and has rolled to windward under the action of waves and then been blown by a gust of wind to leeward. (See Section 3.2 of A.749(18)). In this situation, the regulations describe the total energy of the vessel during the roll to leeward, and compare it with the reserve of energy resisting the roll as the vessel heels further and further to leeward. The energy is described in the following way:
The energy in the vessel when rolling to windward is given by the area a) which is circumscribed by the following three lines:
1. A horizontal line representing the wind gust heeling lever, which is described as 50% greater than the wind heeling lever calculated from the prescribed pressure of the wind acting on the side profile of the vessel,
2. a vertical line representing the angle of roll to windward calculated from a prescribed formula and measured from the angle resulting from the wind heeling lever where it intersects the GZ curve, and
3. the GZ curve between the previously-described two lines.
This area is known as area a.
The energy resisting the vessel roll is given by the area b, which is circumscribed by the following three lines:
1. A horizontal line representing the wind gust heeling lever, which is described as 50% greater than the wind heeling lever calculated from the prescribed pressure of the wind acting on the side profile of the vessel,
2. a vertical line representing the angle at which water starts to flood the vessel and known as the downflooding angle, or 50xc2x0 if this is less than the downflooding angle, and
3. the GZ curve between the previously-described two lines.
The area b under all circumstances must be equal to or at least greater than area a.
The areas a and b are illustrated in FIG. 3.
As can be seen from an examination of FIG. 3, the areas a and b are substantially linked to the value of GM. If GM is decreased, then the GZ curve associated with it is lowered, and the area b is reduced whilst the area a may be increased. As a result of this association, the requirements of the severe wind and weather criterion can usually only be met by having a high value of GM, usually several metres, and considerably greater than the minimum amount allowed by regulation which is 0.15 m. This is particularly onerous for large passenger vessels which typically have large and high superstructures providing a large profile area and hence a large wind heeling lever. This feature increases the area a and decreases area b, and the GM has to be considerably larger than is desirable for such vessels.
This desirability is because the GM value is directly related to the comfort of the vessel. The period taken for the vessel to roll to one side under the action of a wave and then to roll back can be expressed as:
Roll period TR=2KR/GM,
where KR is the transverse polar radius of gyration, and the units are seconds and metres. It can be seen from inspection of the above formula that for a given vessel with a fixed KR, then a high value of GM leads to a correspondingly low value of TR, and a low roll period results in high values of transverse accelerations as the vessel rolls.
Rapid transverse accelerations are directly associated with discomfort for passengers on board a vessel, and therefore to ensure passenger comfort the value of GM must be kept at a low value.
In summary, the severe wind and weather stability criteria dictates a high value of GM, but this in turn results in a higher rolling acceleration and reduced comfort level for persons on board a passenger vessel.
It is possible to manipulate the geometry of the vessel to slightly change the shape of the GZ curve, and hence the areas a and b, which in turn allows for a small reduction in GM. For a vessel having a single hull, which covers the great majority of vessels afloat, one such shape would involve blisters on each side of the craft, which are also called pontoon sides. Another solution, which has been adopted by some designs, involve large overhangs of the ship sides such that the side plating passes through the plane of the water surface at an acute angle, and the ship is considerably wider above the plane of the water than the width at the plane of the water. In this way as the vessel heels it immerses a considerable volume on the submerged side. This partial solution is typical of several large passenger cruise liners.
None of the above solutions are completely satisfactory, because they introduce slamming problems, where the water surface, under the action of waves, impacts on the undersides of the parts that are above the static waterline, creating structural impact loads and creating noise which disturbs the passengers. In addition the effect upon the GZ curve and GM value are not large.
It is practically impossible to reduce the GM value for a vessel having two hulls such as a catamaran, as these craft inherently have very high values of GM owing to the wide separation of the waterplane of the two hulls.
Definitions
The design draught is defined as the position of the waterline at which the vessel is designed to float during the normal operation of the vessel, and may include a range of waterlines depending upon the loading of the vessel and the usage of consumables such as fuel and fresh water. These waterlines may include different trims, where the waterline is not parallel to the baseline of the vessel in the longitudinal direction.
The waterplane of a vessel floating at rest at a draught T is defined as the shape defined by the intersection of the exterior hull shape and a horizontal plane at the water surface. This waterplane will have an area, AWP, and an associated moment of inertia IT about a longitudinal axis running from the bow to the stern on the centreline of the vessel.
In accordance with one aspect of the present invention there is provided a seagoing vessel having a length of between 45 and 175 metres and designed to operate at speeds between 25 and 70 knots, the vessel comprising a single hull with stabilising side hulls (called amahs) positioned on each side of the hull, the ratio of the moment of inertia of the water plane IT to the volume of displacement ∇ (in consistent units) is equal to a value of between 1.0 and 6.0 and the vessel being shaped above the designed water line such that the righting lever (GZ) curve as the vessel heels results in a righting lever (GZ) curve that meets the following requirements:
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Preferably the main hull is designed so that the distance GM determined in the transverse plane for the main hull in isolation and without amahs but floating at a water line equivalent to that for the complete vessel is less than 0.15 metres or negative. The amahs may be designed such that each has a volume of displacement of less than 10%, preferably less than 5% of the total volume of displacement including the main hull.
In accordance with a further aspect of the present invention there is provided a seagoing vessel having a length of between 45 and 175 metres and designed to operate at speeds of between 25 and 70 knots, the vessel comprising a single hull with stabilising amahs positioned on either side, wherein the hydrostatic value of GM determined in the transverse plane lies between 0.5 and 5 metres, the vessel being shaped above the designed waterline such that the righting lever (GZ) curve as the vessel heels meets the following requirements:
the area (b) bounded by the GZ curve plotted on the heeling axis between the angle of flooding and the heeling lever associated with a specific gust of wind is greater in value than the area (a) bounded by the GZ curve plotted on the heeling axis between the heeling lever associated with the specific gust of wind and an angle associated with the amount of roll of the vessel to windward under the action of the waves.